1. Field of the Invention
The present invention relates to a thin film deposition apparatus and, in particular, to a thin film deposition apparatus for depositing thin films by physical vapor deposition (PVD) through which thin films are formed by using ion beams and electron beams.
2. Description of the Related Art
High-quality thin films, such as semiconductors, optical thin films, magnetic films, or insulating films, have previously been formed by sputtering or through low-energy ion-beam assisted deposition.
FIG. 3 schematically shows a conventional thin film deposition apparatus presented, for example, in "Journal of Vacuum Science and Technology (J. Vac. Sci. Technol.), B2(3), Aug. to Sep., 1984". In FIG. 3, a sealed type crucible 3 is disposed in the lower section of a vapor generation source 7 of an ion source apparatus placed inside a vacuum chamber (not shown), the inside of which is maintained at a predetermined vacuum. An orifice 4 is formed above the crucible 3, and a deposition material 5 is housed in the crucible 3. A heater 6 heats the crucible 3. The crucible 3, the orifice 4, and the heater 6 constitute the vapor generation source 7.
A cathode (filament) 10 from which an electron beam is emitted and an anode 11 which attracts electrons from the cathode 10 constitute an ionization means 12. An acceleration electrode 16 and a grounded electrode 17 constitute an acceleration means 19 for accelerating a deposition material ionized by the ionization means 12 in an electric field and providing it with kinetic energy.
With the above-mentioned construction, the vacuum chamber is discharged by a vacuum discharge system until the vacuum chamber attains a vacuum of approximately 106 Torr. After that, the crucible 3 is heated by the heater 6. As a result, the deposition material 5 inside the crucible 3 evaporates and is transported to the ionization means 12 through the orifice 4. The vapor of the deposition material 5 collides with the electron beam emitted from the cathode 10 and is ionized. The ionized vapor of the deposition material 5 is accelerated by an electric field applied by an acceleration electrode 16 of the acceleration means 19 and a grounded electrode 17 and collides with the surface of a substrate (not shown). Thus, a thin film is formed.
Such a conventional thin film deposition apparatus has a problem in that, if a deposition material having a good wettability with a crucible material, such as silicon (Si) or aluminum (Al), is used, a phenomenon such that the molten deposition material creeps up on the side of the orifice or oozes out around the crucible 3, and therefore, the ion source cannot be operated stably. Another problem arises in that since these molten metals undergo a violent reaction with a material forming an ion source, a portion which is in contact with the molten metal corrodes and the lifetime of the apparatus becomes extremely short.
Still another problem is that in a conventional deposition method in which ions of a low energy (20 to 500 eV) are used, the ratio of a generated ion beam to the vapor which it occupies is 5% or less at most, and therefore the ratio of ions used is small, so there is a limitation on the improvement of the performance of thin film devices formed on a substrate.
Still another problem is that even if an attempt is made to control the properties of thin films by the acceleration voltage, the number of ions which reach the substrate is very small, particularly if the acceleration voltage is low, and therefore high-quality thin films cannot be formed when low-energy ions are utilized.
Still another problem is that when the acceleration voltage approaches zero, electrons emitted from the cathode (filament) begin to strike the substrate, and thereby cause damage to the substrate.